Pinholes are used as a component in optical systems both as spatial filters to remove unwanted variations in light intensity across a light beam (typically if the light is generated from a laser) and as alignment aids to insure that incident light impinges on other optical elements at locations determined by the position of the pinhole. Thus, pinholes often must be placed in optical systems at precisely specified locations. A limitation in the usefulness of pinholes is that the exact position of prior art pinholes is difficult to establish initially and subsequently maintain from drifting, making systems which incorporate pinholes as either spatial filters or alignment aids difficult to align and prone to long-term performance degradation as the position of the pinhole drifts.
In a typical prior art pinhole component, the pinhole is laser-drilled through a metal foil. The prior art pinhole is therefore a passive device, incorporating no elements capable of measuring incident light intensity or position and further being incapable of straightforward modification to add position sensing elements. Typical metal foil thickness of 10-12 .mu.m is used for these pinhole devices. However, pinholes smaller in diameter than 5 .mu.m, are typically drilled through 6 .mu.m thick foils. The use of thin metal foils results in fragile components which are prone to damage and warping during normal handling and use.
One embodiment of the present invention includes a thin-film position sensitive photodetector element that is integrated with a pinhole. There are two types of typical prior art position sensitive photodetector elements, each with possible variations on the technique for formation of the photodetector. One is a split-cell type position sensitive photodetector element that consists of multiple planar semiconductor junctions, adjacent but separated by gaps as small as 1 .mu.m. In operation, when light is incident on the split-cell photodetector, different currents will be sensed in each of the semiconductor junctions of the split-cell. A second is a continuous-cell type of position sensitive photodetector element that consists of a single planar semiconductor junction with multiple contacts at its perimeter. In operation, when light is incident on the continuous-cell position sensitive photodetector, different currents will be sensed in each of the perimeter contacts depending on the relative distance between the centroid of the incident light and the perimeter contacts.
Each of the types of position sensitive photodetectors has a different practical application. The split-cell type is most useful for measuring small deviations in the location of an optical beam about a location centered on the gap between elements. The continuous-cell type is most useful for providing an approximately linear change in output signal with the changes in the position of a light beam. This is accomplished by noting the current difference from side to side of the entire photodetector element semiconductor junction. As compared to the spit-cell type, the continuous-cell type measures a larger range of incident light locations with less precision than the split-cell photodetector.
For either type of position sensitive photodetector, the semiconductor junctions are formed as metal-semiconductor junctions (Schottky-type photodetector), p-n semiconductor junctions, or p-i-n semiconductor junctions with the overall device operating as discussed above for any of the junction structures. In the formation of a metal-semiconductor junction, the semiconductor surface is coated with metal (typically less than 10 nm thick). In the p-n semiconductor junction, the manufacturing process introduces a layer, typically less than 1 .mu.m thick, of p-type dopant into an n-type semiconductor substrate (or alternately n-type into p-type). In the p-i-n semiconductor junction, the typical manufacturing process is to epitaxially grow a layer (typically more than 10 .mu.m thick) of lightly doped (near intrinsic) semiconductor on top of an n-type substrate, then introduce a layer of p-type dopant (typically less than 1 .mu.m thick) into the epitaxial layer. In all of the cases described above, the substrate is derived from a semiconductor wafer, and the total device thickness is determined by the thickness of the semiconductor wafer with the wafer thickness being quite variable, i.e. typically 300 .mu.m or more.
In summary, the prior art includes, independently, pinhole plates and position sensitive photodetectors. As discussed above, pinholes are manufactured in thin metal foils with the thin foil making the drilling process possible, and allowing for efficient pinhole operation, however, the pinhole foils are subject to mechanical damage and do not incorporate position sensitive elements. Also, as discussed above, the prior art includes position sensitive photodetectors that are fabricated in relatively thick substrates, hence they are not amenable to combination with the manufacture of a pinhole foil. What is needed is a pinhole and method of manufacture that overcomes the limitations of the presently available art and permits the integration of position sensitive photodetectors with pinhole assemblies. The present invention provides such a device and method of manufacture.